I would like to achieve that every entry provided by example.org
server have extra attribute "mailhost: smtp.example.org" and every
entry provided by example.com have extra attribute "mailhost:
mail.example.com". Adding such attributes to primary ldap servers is
not an option.

Not out of the box, and this is by design: back-ldap/back-meta are
designed to provide simple naming context virtualization by default,
while the rewrite/remap capability gives further mucking capabilities
but on DNs only. You need to write an overlay that mucks with
searchEntry intermediate results; I'm not sure you can easily get that
info - that is, who returned the entry, unless the entry's content can
help you.

If I would extend meta backend to optionally append operational
attribute sourceURI (or any better name), then maybe it would be easier
to get to reporting server information in the overlay. Or would such
extension ruin design too?